The present invention relates to an improvement in the preparation of a polyvinyl chloride resin by the suspension polymerization of vinyl chloride monomer in an aqueous medium or, more particularly, to an improvement in the preparation of a polyvinyl chloride resin having well developed porosity of the resin particles to exhibit good plasticizer absorptivity and containing a greatly decreased amount of fish eyes by the suspension polymerization of vinyl chloride monomer in an aqueous medium.
Various proposals and attempts have been made heretofore relative to the method for the preparation of a polyvinyl chloride resin having good workability in resin molding and containing a decreased amount of fish eyes by the suspension polymerization of vinyl chloride monomer in an aqueous suspension medium. For example, Japanese Patent Kokai 52-5886, 52-110797, 52-115890, 55-112210 and 53-6392 and Japanese Patent Publication 61-18562 each propose a method in which a partially saponified polyvinyl acetate of a low degree of saponification of, e.g., 55% or lower, and a low average degree of polymerization of, e.g., 600 or lower, which is also called an oil-soluble polyvinyl alcohol, is used as a dispersing agent of the monomer so that the polyvinyl chloride resin obtained thereby has well developed porosity to exhibit improved workability and contains a greatly decreased amount of unreacted residual monomer absorbed in the resin particles.
The above described prior art method is indeed effective at least to some extent in increasing the porosity of the resin particles but a disadvantage is unavoidable therein that the bulk density of the resin powder is more or less decreased. When the amount of the so-called oil-soluble polyvinyl alcohol is increased with an object to further increase the porosity of the resin particles, in particular, an adverse influence is caused on the stability of the suspension resulting in occurrence of coarse particles in the resin product which may eventually contain agglomerates of resin particles along with a problem of an increase in the amount of polymer scale deposited on the walls of the polymerization reactor.